Pierre,
Depending on the object "myLastFile", the method openDlg might not even exist (if the myLastFile object is not a File object, for instance). And I do not see any need for the myLastFile anyhow, as you are presenting a dialog to select a file to open. I recommend using the global ChooseFile( ) method instead. This will give you a filename as string in full path notation, or null when no file was selected in the dialog. I am not sure what your ExtendScript documentation states about the return value for ChooseFile, but if that differs from what I am telling you here, the documentation is wrong. So, if you replace the first lines of your code with the following it should work:
function openXMLFile ( ) {
var filename = ChooseFile ( "Choose XML file ...", "", "*.xml", Constants.FV_ChooseSelect );
While writing this, I see that Russ has already given you the same advice. Use the symbolic constant value I indicated to use the ChooseFile dialog to select a single file (it can also be used to select a directory or open a file - but you want to control the opening process yourself). Note that this method allows you to set a start directory for the dialog (second parameter). The ESTK autocompletion also gives you a fifth parameter "helplink" which is undocumented and can safely be ignored.
Good luck
Jang